From: Florian Fainelli
> Sent: 09 November 2017 22:35
>
> In preparation for enabling Broadcom tags with b53, pad packets to a
> minimum size of 64 bytes (sans FCS) in order for the Broadcom switch to
> accept ingressing frames. Without this, we would typically be able to
> DHCP, but not resolve with ARP because packets are too small and get
> rejected by the switch.
>
> Signed-off-by: Florian Fainelli <[email protected]>
> ---
> drivers/net/ethernet/broadcom/bgmac.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/drivers/net/ethernet/broadcom/bgmac.c
> b/drivers/net/ethernet/broadcom/bgmac.c
> index 48d672b204a4..5130fc96940d 100644
> --- a/drivers/net/ethernet/broadcom/bgmac.c
> +++ b/drivers/net/ethernet/broadcom/bgmac.c
> @@ -127,6 +127,8 @@ bgmac_dma_tx_add_buf(struct bgmac *bgmac, struct
> bgmac_dma_ring *ring,
> dma_desc->ctl1 = cpu_to_le32(ctl1);
> }
>
> +#define ENET_BRCM_TAG_LEN 4
> +
> static netdev_tx_t bgmac_dma_tx_add(struct bgmac *bgmac,
> struct bgmac_dma_ring *ring,
> struct sk_buff *skb)
> @@ -139,6 +141,16 @@ static netdev_tx_t bgmac_dma_tx_add(struct bgmac *bgmac,
> u32 flags;
> int i;
>
> + /* The Ethernet switch we are interfaced with needs packets to be at
> + * least 64 bytes (including FCS) otherwise they will be discarded when
> + * they enter the switch port logic. When Broadcom tags are enabled, we
> + * need to make sure that packets are at least 68 bytes
> + * (including FCS and tag) because the length verification is done after
> + * the Broadcom tag is stripped off the ingress packet.
> + */
I think that that would be better as:
/* Ethernet packets are padded to 64 bytes (including FCS).
* If 'Broadcom tags' are enabled they must still be 64 bytes
* long after the 4 byte tag is removed.
* Since the hardware doesn't do it, we must pad them before
* transmit.
*/
Which seems to be to be a bug in the chip.
> + if (skb_put_padto(skb, ETH_ZLEN + ENET_BRCM_TAG_LEN))
> + goto err_stats;
> +
But you shouldn't overpad packets that don't have the extra tag.
David
